"Wireless cable" is a term usually used to refer to a multi-channel video distribution medium that resembles franchise cable television, but which uses microwave channels rather than coaxial cable or wire to transmit programming to the subscriber. Programming for wireless cable systems is received at the headend of the wireless cable system in the same manner as it is for landline based cable television. These programs are then retransmitted, utilizing the Ultra High Frequency (UHF) portion of the microwave radio frequency spectrum (2.1 to 2.7 GHz), by a microwave transmitting antenna located on a tower or other tall structure to small antennas on subscriber rooftops, typically within a 40 mile radius.
In a typical prior art system, such as shown in FIG. 1, a headend system H receives up to a maximum of 33 analog television program signals from a variety of satellite down-link receivers and other types of receivers, in the exact same manner as for a cable television system. The headend system H frequency multiplexes those television program signals into a combined spectrum signal in the 50-450 Mhz range. This combined signal has a frequency distribution similar to that found on a cable television network. The headend system upconverts the combined spectrum signal to the UHF frequency range, typically centered around 2.6 Ghz. The headend system supplies the UHF signal to a single transmitter antenna tower T which broadcasts the signal to subscribers who each have an individual home receiving system. Subscribers can call in to the headend to order pay-per-view events via the telephone network, and the headend transmits codes to the subscribers systems to enable descrambling of encoded pay-per-view programs.
FIG. 1A shows a typical service area for a wireless cable type system of the type shown in FIG. 1. In accord with relevant regulations, a multi-channel multi-point distribution service (MMDS) type wireless cable operator has a protected or `primary` reception area P. At the relevant frequencies here under consideration, the primary area P is a circle having a radius of 15 miles from the operator's transmitter T1. Within this area, the operator is guaranteed that there will be no interference with his transmissions on the assigned frequency channel(s). However, at the allowable power levels, the transmissions from antenna tower T will propagate out over a secondary area S having a radius of up to 40 miles. Within the secondary area, some locations will receive sufficient signal strength to utilize the wireless cable services.
Within a selected geographic area, one or more educational institutions may also have licenses to operate an Instructional Television Fixed Service (ITFS) system. Under regulations relating to ITFS, such a service provider has a protected area defined by the locations of the receivers. For example, if the educational institution is a parochial school system operating 7 schools within the geographic area, there would be one receiver R (indicated by circles in FIG. 1A) at each school. The school system would operate a broadcast transmitter T2 on the allocated channel(s). The regulations guarantee protection from interference on the channel(s) only within the zone defined by the receivers (indicated by the dotted lines between the small circles). Another educational institution may operate a similar transmitter T3 and its own set of receivers in another part of the geographic area using a different block of UHF channels. The ITFS service areas may overlap or be contained entirely in the geographic are corresponding to the primary area of MMDS service or the secondary area of MMDS service.
UHF signals in the relevant frequency band arrive at a receiver location by direct line-of-sight (LOS) transmission. Typically an elliptical dish shaped antenna 18-36 inches long, formed of parallel curved elements, is aimed from the subscriber location to receive the strongest signal from the transmitter. The captured signals are down-converted at the antenna from the microwave band to the broadcast band and transmitted via coaxial wiring into the house. For scrambled signals (the typical case), a set top converter functionally similar to a cable set top box is used. In many UHF installations, to conserve UHF capacity for premium services, a VHF/UHF off-air broadcast receive antenna is installed with the UHF antenna to pick up the local programming.
To a user or subscriber, wireless cable operates as a cable look-alike service. Because wireless cable signals are transmitted over the air rather than through underground or above-ground cable networks, wireless systems are less susceptible to outages and are less expensive to operate and maintain than franchise cable systems. Most service problems experienced by wireless cable subscribers are home-specific rather than neighborhood-wide, as is frequently the case with franchise cable systems.
The evolution of wireless cable may be briefly summarized as follows. Wireless cable technology has existed in a single channel version for commercial purposes since the 1970's and had been available even longer for educational use. In mid-1983, the FCC, invoking the need to promote competition with conventional cable television systems, established a change in the rules for using a portion of the microwave spectrum previously designated for educational use. In the past, 28 microwave channels had been available to accredited and non-profit educational organizations for educational use exclusively by Instructional Television Fixed Service (ITFS) operators. Rules reallocated eight of those channels for outright commercial use, and educational organizations were permitted to lease excess hours to commercial operators on the remaining 20 channels. In any local market, this makes it possible for a commercial operator to combine any or all of those 28 channels with five other channels already available for commercial use. Under current FCC rules, the available spectrum results in a maximum of 33 analog channels. This number of `wireless cable` channels is less than the number offered on many competing franchise type cable television systems.
Since 1983 spectrum blocks in the 2.1-2.7 GHz range have been allocated for the purpose of delivering video content from a single transmit site to multiple receive locations. A total of 198 Mhz has been allocated for downstream transmission for the wireless cable service. The channelization and transmission modulation (6 Mhz amplitude modulation/vestigial side band) are equivalent to broadcast TV or cable but up-converted to microwave frequencies.
The 33 channels potentially available to wireless cable operators are subdivided into two types of channels. Twenty channels are referred to as ITFS. The remaining 13 channels are generally referred to as Multi-channel Multipoint Distribution Service (MMDS).
The current UHF spectrum was originally licensed in blocks of four video channels each separately licensed, with each block allocated to a specific purpose. Five groups, each with four channels, were allocated to Instructional Television Fixed Service (ITFS). ITFS spectrum was initially made available only to educational institutions. Two groups of four channels were made available to anyone wishing to provide an alternative multi-channel video program service. The final four channels were licensed individually to institutions for the purpose of providing a private video network. Over time, the FCC relaxed some of these operational rules. Through licensing and leasing arrangements, the FCC now allows all of the channels to be aggregated for the purpose of providing an alternative to franchise cable television. However, even in areas where it is possible for one operator to aggregate the necessary licenses, the system capacity is still limited, i.e. to 33 channels or less.
In many ways, current typical UHF wireless TV is equivalent to at most a low tier franchise cable television system (i.e. having relatively few channels). Other than the number of program channels, the only real difference arises in the medium used to transport signals from the headend to the customer. Functionally identical headend equipment is utilized in both systems. In the case of UHF service, signals leave the headend via a microwave transmitter. With cable television, the same signals leave the headend on fiber or coaxial cable facilities. However, wireless cable systems have had difficulty competing because today many cable systems offer a more diverse range of programs.
Also, propagation characteristics at the relevant UHF operating frequencies require line-of-sight (LOS) between the transmit and receive antennas for reliable service reception. Both natural obstructions such as hills and vegetation, and man-made obstructions such as buildings, water towers and the like, limit the actual households capable of receiving an LOS transmission. One solution to the blockage problem has been to provide repeaters. A repeater receives the primary transmission from tower T on the tower side of the obstruction, amplifies the signal if necessary, and retransmits the signal into the area of blockage. This may be an effective solution to one blockage or obstruction, but in many major metropolitan areas there are many obstructions. The power levels permitted for such repeaters tend to be low, and overcoming blockages due to distortions that result when amplifying combined RF channels caused by many different obstructions to the primary transmissions would require an inordinate number of repeaters. Also, because of delays and multipath effects, repeater transmissions may interfere with reception from the primary source in areas close to the blockage area B.
In the industry, a nominal figure for households reachable by LOS is 70%, even with a small, commercially practical number of repeaters. This projected number is based solely on computer models, not actual field measurements. It is believed that actual coverage by the current wireless cable technology in the UHF medium is considerably lower. Typical antenna heights required to achieve the present level of coverage in commercial service are 800-plus feet for transmitters and 30-60 feet for receivers. That means that many receive antennas must be mounted atop masts or nearby trees as an alternative to a rooftop mounting. While current regulations provide a 15 mile protected service area for MMDS, it is desired that effective system coverage for approximately 40-70% of the affected households may be achieved to a 40 mile radius from the transmitter antenna.
Besides signal blockage, several other propagation factors can affect reliable UHF service delivery. One factor is multi-path reflections of the desired signal arriving at the receiver by way of differing paths and therefore arriving with slight delay. For analog video signals, multi-path appears as ghost images on the viewer's TV. For digital signals, multi-path can cause intersymbol interference that results in multiple bit errors. In either case, near-coincident multi-path signals can cause a degree of signal cancellation that looks like additional propagation loss. Multi-path also results from reflections and diffraction.
Path fading is another significant coverage factor. Time-variant path fading can result from atmospheric effects, e.g., rain or temperature and pressure inversions. Rain can act to partially reflect or absorb the microwave signals. Weather inversions can result in an upward bending of the wave front due to refraction. There are engineering measures to mitigate the troublesome effects of time-variant path fading, such as suitable fade margins and antenna diversity.
Clearly a need exists for a wireless cable type broadcast system providing an increased channel capacity, without requiring additional spectrum allocation. In the market areas where existing channel licenses are held by an instructional institution, obtaining the license often requires that the would be new operator agree to provide the institution with transport capacity at least equivalent to the capacity that the institution is using on the analog system. It therefore is desirable to transport an existing provider's programming (e.g. from the institution), and the increased capacity should be sufficient to offer a commercially attractive number of new programs (e.g. as many or more than provided via competing landline cable television systems operating in the relevant market area).
Any wireless system providing the increased program transport capacity also should provide increased propagation coverage and reduced areas of blockages. The system should provide good signal quality throughout the entire reception area or service area. Accordingly, it is also desirable to minimize multipath interference and loss of service due to fading.
Disclosure of the Invention
To achieve the above stated objectives, the present invention comprises methods for upgrading the program transport capacity of an RF broadcast channel. The first step of such a process is acquiring access to one or more RF broadcast channels. Typically, existing licensees or service providers are using each of these channels to carry some form of analog program service, e.g. an instructional video service. Once rights to use the channel have been acquired, a digital multiplexing system is deployed for broadcasting a digital transport stream over the RF broadcast channel. The digital transport stream will include compressed digital data representing information from the program service formerly carried on the analog channel. The provider therefore can use this transport capacity to continue to offer the existing service. The broadcast transport stream also includes compressed digital data representing one or more new program services.
The preferred embodiment typically achieves at least a four to one increase in the number of programs transported. In an RF channel that carried one analog program, the digital compressed transport stream carries at least four programs. Although adaptable to audio only programs or data transport, the preferred embodiment provides transport for audio and video programming, typically for television type presentation to users.
In operation, a user of the existing service will obtain a digital receiver; and through that receiver, she will continue to receive the existing service. These users may also receive one or more of the new program services. The new services transported on the digital broadcast system and possibly the existing service may now be offered to new users. Because of the increased number of programs carried on the system, the new program services alone or in combination with the existing service are more attractive to end users or potential subscribers.
To overcome the other problems encountered in wireless cable type broadcasting services, the digital broadcasting system utilizes an RF frequency simulcasting method for transmitting information from multiple spaced transmitting sites to multiple receiving sites in a reception area. A signal including multiplexed channels is transmitted simultaneously from a plurality of spaced transmitting sites. The transmitting sites propagate the signal into substantially overlapping regions of at least a major portion or the intended reception area. At the subscriber premises, a terminal device receives a signal containing a plurality of time delayed versions of the transmitted signal. At least a portion of the received signal is processed to acquire a single representation of a signal corresponding to a selected one of the multiplexed channels, and information contained in the acquired signal is presented, e.g. in a form that is sensorially perceptible to a user. In the preferred embodiments, the processing of multiple copies utilizes a delay equalization technique.
The overlapping transmission or propagation areas reduce or eliminate blockage zones and effects of fading. This technique also limits the number of receivers effected by equipment outages. Typically, a directional receiving antenna can be aimed toward at least one strong line-of-sight transmission source.
In a television type broadcasting system, each program is digitally encoded into compressed digital data. A multiplexer combines the compressed digital data for a group of programs into one of the digital multiplexed data streams for transport through one of the channels. The presentation of one program to a user entails selecting one of the channels, selecting data from the multiplexed stream of digital data carried in that channel, and reproducing the selected data, e.g. as an audio/visual output through a television set.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.